1. Field of the Invention
The field of the invention is data processing, or, more specifically, methods, systems, and products for broadcasting a message in a parallel computer.
2. Description of Related Art
The development of the EDVAC computer system of 1948 is often cited as the beginning of the computer era. Since that time, computer systems have evolved into extremely complicated devices. Today's computers are much more sophisticated than early systems such as the EDVAC. Computer systems typically include a combination of hardware and software components, application programs, operating systems, processors, buses, memory, input/output devices, and so on. As advances in semiconductor processing and computer architecture push the performance of the computer higher and higher, more sophisticated computer software has evolved to take advantage of the higher performance of the hardware, resulting in computer systems today that are much more powerful than just a few years ago.
Parallel computing is an area of computer technology that has experienced advances. Parallel computing is the simultaneous execution of the same task (split up and specially adapted) on multiple processors in order to obtain results faster. Parallel computing is based on the fact that the process of solving a problem usually can be divided into smaller tasks, which may be carried out simultaneously with some coordination.
Parallel computers execute parallel algorithms. A parallel algorithm can be split up to be executed a piece at a time on many different processing devices, and then put back together again at the end to get a data processing result. Some algorithms are easy to divide up into pieces. Splitting up the job of checking all of the numbers from one to a hundred thousand to see which are primes could be done, for example, by assigning a subset of the numbers to each available processor, and then putting the list of positive results back together. In this specification, the multiple processing devices that execute the individual pieces of a parallel program are referred to as ‘compute nodes.’ A parallel computer is composed of compute nodes and other processing nodes as well, including, for example, input/output (‘I/O’) nodes, and service nodes.
Parallel algorithms are valuable because it is faster to perform some kinds of large computing tasks via a parallel algorithm than it is via a serial (non-parallel) algorithm, because of the way modern processors work. It is far more difficult to construct a computer with a single fast processor than one with many slow processors with the same throughput. There are also certain theoretical limits to the potential speed of serial processors. On the other hand, every parallel algorithm has a serial part and so parallel algorithms have a saturation point. After that point adding more processors does not yield any more throughput but only increases the overhead and cost.
Parallel algorithms are designed also to optimize one more resource the data communications requirements among the nodes of a parallel computer. There are two ways parallel processors communicate, shared memory or message passing. Shared memory processing needs additional locking for the data and imposes the overhead of additional processor and bus cycles and also serializes some portion of the algorithm.
Message passing processing uses high-speed data communications networks and message buffers, but this communication adds transfer overhead on the data communications networks as well as additional memory needed for message buffers and latency in the data communications among nodes. Designs of parallel computers use specially designed data communications links so that the communication overhead will be small but it is the parallel algorithm that decides the volume of the traffic.
Many data communications network topologies are used for message passing among nodes in parallel computers. Such network topologies may include for example, a tree, a rectangular mesh, and a torus. In a tree network, the nodes typically are connected into a binary tree: each node typically has a parent and two children (although some nodes may only have zero children or one child, depending on the hardware configuration). A tree network typically supports communications where data from one compute node migrates through tiers of the tree network to a root compute node or where data is multicast from the root to all of the other compute nodes in the tree network. In such a manner, the tree network lends itself to collective operations such as, for example, reduction operations or broadcast operations. The tree network, however, does not lend itself to and is typically inefficient for point-to-point operations.
A rectangular mesh topology connects compute nodes in a three-dimensional mesh, and every node is connected with up to six neighbors through this mesh network. Each compute node in the mesh is addressed by its x, y, and z coordinate. A torus network connects the nodes in a manner similar to the three-dimensional mesh topology, but adds wrap-around links in each dimension such that every node is connected to its six neighbors through this torus network. In computers that use a torus and a tree network, the two networks typically are implemented independently of one another, with separate routing circuits, separate physical links, and separate message buffers. Other network topology often used to connect nodes of a network includes a star, a ring, or a hypercube. While the tree network generally lends itself to collective operations, a mesh or a torus network generally lends itself well for point-to-point communications. Although in general each type of network is optimized for certain communications patterns, those communications patterns may generally be supported by any type of network.
In many of these data communications networks, transfers between source and target nodes generally supports a deposit mechanism that allows a copy of the network packet, as that packet travels along a network axis from the source node to the target node during that transfer, to be provided to each intermediate compute node on that axis between the source node and the target node. That is, the deposit mechanism is so called because a copy of the packet is deposited on each intermediate node between the source and the target node along the same network axis. Employing the deposit mechanism differs from when the deposit mechanism is not used because the only node that receives the network packet for processing is the target node.
Using a deposit mechanism in a three-dimensional rectangular mesh or torus network allows a node to broadcast a network packet to all of the nodes in the network in at least three phases. During the first phase, a compute node broadcasts the packet to all the nodes along an axis in a first dimension of the network. During the second phase, each compute node that has the packet broadcasts the packet along an axis of the second dimension perpendicular to the first dimension. After the second phase, therefore, an entire plane of nodes along the first and second dimensions in the rectangular mesh or torus network has received the network packet. During the third phase, each compute node that has the packet broadcasts the packet along an axis in the third dimension of the network. After the third phase, therefore, all of the nodes in the network have a copy of the network packet for processing. The drawback to this three phase approach, however, is that after each phase, the nodes must synchronize before proceeding to the next phase because some nodes receive the network packet before other nodes in the same phase. For each phase of the broadcast, therefore, some nodes are idle, which introduces synchronization overhead into the system and limits parallelism.